Bohr's model: fixed orbits and energy levels
Bohr said electrons move only in fixed orbits, or energy levels, without losing energy. An electron changes level only by absorbing or giving out the exact energy difference.
Bohr's two postulates (1913)
- Postulate 1: electrons move around the nucleus only in certain fixed circular orbits, each with a fixed energy, like planets around the Sun. These are called orbits, energy levels or stationary states.
- An electron in such an orbit gives out no energy, so it does not slow down or fall into the nucleus.
- Postulate 2: an electron moves to a higher level by absorbing energy, and drops to a lower level by giving out energy.
- The energy taken in or given out is one packet of light, a photon. It equals the difference between the two levels: E = hν = E₂ − E₁. Here E₂ is the higher level and E₁ the lower; h is Planck's constant and ν (nu) is the frequency of the light.
How Bohr fixed Rutherford's problems
Rutherford's electron should lose energy and spiral into the nucleus. Bohr said that in an allowed orbit it does not lose energy. So the atom stays stable. Bohr also gave each shell a fixed number of electrons, so his model shows how the electrons are arranged. The third drawback, the extra mass of atoms, was solved later by the discovery of the neutron.
Shells and how many electrons they hold
| n | Shell | Energy | Most electrons (2n²) |
|---|---|---|---|
| 1 | K | Lowest | 2 × 1² = 2 |
| 2 | L | Higher | 2 × 2² = 8 |
| 3 | M | Higher still | 2 × 3² = 18 |
| 4 | N | Higher still | 2 × 4² = 32 |
Orbit or shell?
How to draw Bohr's model (energy levels)
- Draw a small circle in the centre labelled nucleus (+).
- Draw four circles around it, spaced outwards. Label them K (n = 1), L (n = 2), M (n = 3) and N (n = 4).
- Beside each circle, write how many electrons it can hold: 2, 8, 18, 32.
- Draw an arrow pointing outwards labelled increasing energy.
- To show a jump, draw an arrow from M down to L with a wavy line beside it labelled photon given out, E = hν.
Diwali fireworks glow in colours because heat lifts electrons in metal atoms to higher levels. As they drop back, each metal gives out its own colour: strontium red, barium green and sodium yellow.
2 marks: “How did Bohr's model explain why atoms do not collapse?” Bohr said electrons move only in fixed orbits (stationary states) of definite energy, and an electron in such an orbit gives out no energy. So it does not slow down or spiral into the nucleus, and the atom stays stable.
1 marks: “What is the largest number of electrons the M shell can hold?” 2n² = 2 × 3² = 18.
3 marks: “With a labelled diagram, explain what happens when an electron in an atom absorbs energy and then returns to its original level.” It jumps to a higher level by absorbing exactly the energy difference. It returns by giving out the same energy as light, E = hν = E₂ − E₁. Diagram: nucleus with labelled K, L and M levels, an upward arrow (absorb) and a downward arrow with a wavy line (light given out).
Bohr's rule: stay in your lane and lose nothing; change lanes only by taking in or giving out the exact difference.